Changes in temperature and precipitation extremes in the IPCC ensemble of global coupled model simulations

Temperature and precipitation extremes and their potential future changes are evaluated in an ensemble of global coupled climate models participating in the Intergovernmental Panel on Climate Change ( IPCC) diagnostic exercise for the Fourth Assessment Report ( AR4). Climate extremes are expressed in terms of 20- yr return values of annual extremes of near- surface temperature and 24-h precipitation amounts. The simulated changes in extremes are documented for years 2046 - 65 and 2081 - 2100 relative to 1981 - 2000 in experiments with the Special Report on Emissions Scenarios ( SRES) B1, A1B, and A2 emission scenarios. Overall, the climate models simulate present- day warm extremes reasonably well on the global scale, as compared to estimates from reanalyses. The model discrepancies in simulating cold extremes are generally larger than those for warm extremes, especially in sea ice - covered areas. Simulated present- day precipitation extremes are plausible in the extratropics, but uncertainties in extreme precipitation in the Tropics are very large, both in the models and the available observationally based datasets. Changes in warm extremes generally follow changes in the mean summertime temperature. Cold extremes warm faster than warm extremes by about 30% - 40%, globally averaged. The excessive warming of cold extremes is generally confined to regions where snow and sea ice retreat with global warming. With the exception of northern polar latitudes, relative changes in the intensity of precipitation extremes generally exceed relative changes in annual mean precipitation, particularly in tropical and subtropical regions. Consistent with the increased intensity of precipitation extremes, waiting times for late- twentieth- century extreme precipitation events are reduced almost everywhere, with the exception of a few subtropical regions. The multimodel multiscenario consensus on the projected change in the globally averaged 20- yr return values of annual extremes of 24- h precipitation amounts is that there will be an increase of about 6% with each kelvin of global warming, with the bulk of models simulating values in the range of 4% - 10% K-1. The very large intermodel disagreements in the Tropics suggest that some physical processes associated with extreme precipitation are not well represented in models. This reduces confidence in the projected changes in extreme precipitation.